Process for flattening thermistor flakes



Sept. 1, 1959 R. w. HALL 2,901,811

PROCESS FOR FLATTENING THERMISTOR FLAKEs Filed oct.' s1, 195s a@ /4 gj cg Ric/Lard WCLZZ ATTORNEY PROCESS FOR FLATTENING THERMISTOR FLAKES i Richard W. Hall, Springdale, Conn., assignor to Barnes Engineering Company, Stamford, Conn., a corporation of Delaware Application October 31, 1955, `Serial No.-543,672 s claims. (C1. zs``1s6 This invention is concerned with an improved process for shaping thermally sensitive elements. More particularly, it concerns an improved process for shaping or Hattening Hakes of thermally sensitive resistance material used to detect infra-red radiation in thermistor bolometers within very exacting tolerances.

Thermistor bolometers, which are electrical devices sensitive to infra-red radiation use small, thin Hakes of thermally sensitive resistance material to `detect impinging infra-red energy. In general the Hakes are cemented to the Hat surface of a block of thermally conducting, electrically insulating material which conducts heat away from the Hake, and is therefore known as a thermal sink. To obtain a radiation sensitive unit with fast response speed, the cement layer which joins the Hake and backing block should be of minimum thickness. However, when Hakes are made according to conventional processes they are curved or bowed at the edges, and if joined to a backing block with the thinnest layer of cement possible at the central portion thereof, the upward curl of the edges requires a much thicker cement layer. If the Hakes are placed on the block with their edges curled downwardly, the thick cement layer is at the central portion. Thus this Hake curvature results in cement layers whose average thickness is approximately two or three times the minimum possible thickness if the Hake is Hat. Since the thermal conductivity of the cement layer is approximately 3A0() that of the average backing block, this condition causes slow response speeds for units made from unHattened Hakes because of greater thermal resistance between the Hake and the block. A further disadvantage in using curled Hakes lies in the resulting uneven heat conduction from the Hake. Where the cement layer is very thin in the central portion, heat is conducted away rapidly while at the thick edges conduction is comparatively slow. In certain applications the Hakes may be sandwiched between two layers of material, one or both of which may be transparent to infra-red energy, in which event it is also desirable to obtain very Hat Hakes to minimize the thickness of the adjacent cement layers. Other applications require Hakes mounted on optically curved or shaped surfaces. Flakes made by usual techniques must be re-shaped to' fit these surfaces and this invention is directed to such re-shaping as well as Hattening.

Thermistor Hakes are made by mixing in a ball mill metallic oxides, for example oxides of manganese, nickel, iron, cobalt, a plasticizer and a solvent. After sufficient mixing, the material is removed from the mill and spread to dry on glass plates. When dry, the sheet so formed is cut into small flakes and placed on Alundum plates in a furnace to sinter at a high temperature driving olf the solvent and bonding the oxides and plasticizer into a coherent material. Although the Hakes are Hat when entering the sintering furnace, the high temperatures encountered therevcause their edges to curl and bodies to warp. For example, in a standard Hake 10 microns rfvice thick, this curvature may generally range between l5 and 200 microns measured from the plane of the lower surface of the Hake to the lower surface of the curled edge. As noted above such curvature is unacceptable if the Hakes are to be used on Hat surfaces. The amount of curvature which is acceptable is of theorder of 5 to 10 microns, but this is rarely found in Hakes removed from the sintering furnace. Where the Hakes are curved to bebonded to a curved surface they'are seldom curved to proper shape or degree when made according to present practices. Obviously if the Hakes are notshaped to fit the curved surface to which they are to be bonded, the attaching cement layers will be non-uniform in thickness; the result is deterioration in bolometer performance.

Attempts have been made to Hatten curled Hakes by sandwiching them between Hat-surfaced Alundum plates and reheating the Hakes to a high temperature. However, this process results in great breakage principally because the Alundum plates and the Hakes have different coefficients of thermal expansion. Further, Alundum particles break loose from the plates and adhere to the Hakes causing some deterioration in Hake performance. Flakes so Hattened are usually not Hat enough to be acceptable for use with high speed thermistor bolometers.

Accordingly it is an object of this invention to provide an improved method of shaping Hakes to correspond to the surface to 'which they are to be attached. Another object is to provide a method of the above character by which Hakes of various shapes may be made to iit curved or irregularly shaped surfaces. A still further object is to provide a method ofthe above character for making Hat Hakes in production quantities simply and economically. A inal object is to provide a method of the above character for making shaped Hakes wherein appreciable breakage of the Hakesis eliminated. Other objects of the invention will be in part obvious and in part appear hereinafter. .l

This invention accordingly comprises the several steps and the relation of-one or more such steps with respect to each of the others thereofwhich will be exemplified in the method hereinafter disclosed, and the scope of the invention ,will be indicatedin the claims. l

ForV a fuller understanding of `the nature and objects of the invention, reference should -be had to the following detailed vdescription-taken inconnection with the accompanying drawing in which' the single figure is a vertical cross-.section of afurnace which can be used in the Hattening process of this invention.Y

In making the Hakes, a mixtureof oxides, a solvent, and an organic plasticizer is prepared. The preferable oxides are either a mixture yof manganese oxide and nickel. oxide, or a mixture of manganese oxide and nickel oxide ywith cobalt oxide although other materials might be used. The oxide powder 'is mixed with an organic plastic material such as polyvinyl butyral cement, a Hexible thermo-plastic resin which is diluted with a solvent such as methyl ethyl ketone. This mixture lis poured intoa container to which are added steel balls and the Whole is rotated or milled for several hours.

When mixing is substantially complete and the particles are sufficiently dispersed in the reduced plastic, the material is vspread on a glass plate with a doctor blade and allowed to dry. VVWhen sufficiently dry to form laV coherent sheet, the material is dampenedwith distilled water, peeled off the glass/plate and allowed to dry on absorbent paper. The film is then cutto the desired Hake size with a guillotine ,type -microtomesimilar to those used in slicing tissue samples. The Hakes are rcut 20 to 30% larger than the Hnal desired size ton allow for shrinkage `during sintering. Such-Hakes are laid on an Alundum plate and placed in a furnace for sintering. .The furnace is raised to ay high temperature,` approximately l.125(l" C.,

v3 and held therefor-a few minutesyit'isthen dropped to 850 C. for about one hour. After cooling, the sintered Hakes are coherent and fragile, but as previously explainedftheyfarecurled tov-such'an'extent thatthey are not-usable high perforance bolometers, but must be shaped for this purpose.

The vfirst step in the shapingprocess is -to clean the Hakes. This is accomplished by placing them on a vacuum chuckand cleaning Alundum particles from the Hakes with a camels hair brush or small wire.` They may also be cleaned by cooking-in butyl alcohol for a short period, or agitated, drained and allowed to dry. If the particles are left on the-Hakes they may damage the Hakes themselves'as well as the plates on which the Hakes are laid during the Hatteningk process.

After cleaningfthe Hakes are laid out on ground and polished Hat plates-approximately one inch by oneinch by la inch. These-plates maybe glass, Aor other material capable of withstanding the high temperature rto which the Hakes are subjected vin the shaping process. I have found that such materials as a glass composed of 96% silica and4% boron, a glass of '100% silica, or synthetic sapphire (crystalline A1205) are suitableforthis purpose. These materials-are preferred over Alundum for this use since they have a coefficient of thermal expansionsimilar to the Hake material and can be ground and polished Hat or in a variety of shapes with a relatively smooth surface.

A plurality of Hakesare .placed on each A.plate and a second plate is 4then placed on the Hakes to be supported by the curled Hakes. Thus `a suHicie'nt number .of .Hakes must be placed on each plate so that the upper plate will be supported without Hake breakage. A method of placing these plate-Hake sandwiches in a furnace is illustrated in the drawing wherein a cubical enclosure of refractorymaterial generally indicated at is surrounded on the top and sides ,by an inner cover 12 and an outer cover 14 both covers being made from'a high temperature metal alloy. The bottom 16 of enclosure 10 is mounted o-n plate 18 by way of ceramic spacers 20, plate :1S .forming the bottom for inner metal cover 12. Plate '18 in turn is mounted on -plate .22 which forms the vbottom ufor outer cover 14, and is separated therefrom by ceramic spacers ,24. Plate 22fismounted on a transite base 26 by spacers 28- Top 30 and sides 32.0f ceramic cube 10 are fastened together and attached by ceramic spacers 34 to inner cover 12, which in turn is attached by ceramic spacers 36 to outer cover 14.

By constructing 'the furnace in this fashion, the top and sides of the refractory ycubical enclosure, and the covers associated-therewith may be lifted from the bottom 16, to expose the interior "ofrthe furnace thus permitting access thereto. Although air in the space between inner metal cover 12 and outer cover 14 provides some insulation for the furnace, the space is purposely not filled with better insulating material to insure that the furnace may be rapidlyheated and cooled Vto achieve fast Hake production. Guide rods 38 attached to the sides ofouter cover 14 .interlit with holes win base 26 kto position the cover 14, `and parts attached thereto when placing the cover in position. The bottom 16, top 30 and sides 32 of the cubical enclosure all 'have wires 42 lon their interior surfaces 16a, 30a and 32a connected to a source of electrical energy (not shown) toheat the furnace interior.

In the furnace interior spacers 44 support an Alundurn plate 46 in which there 'is a hole 46a above the bottom 16 of the furnace. A thin platinum plate 48 separated from plate V46 yby spacers 50 acts as a radiation shield for a thermocoup-le 52 attached to its llower surface 48a and the wires 54 which connect the thermocouple 52 to an electrical circuit (not shown) are led out through the hole 46a-in the plate 46, hole `16h andthe Iholes 18a, 22a and 26a in the'plates '18 yand 22 in base k26 respectively. A 'lower kglass plate 56 is separated by spacers 58 from the platinum radiation shield'48. A plurality Qf. Quilt-Cl Hakes 60 are arranged on the upper surface 56a of the glass plate 56 and a second glass plate 62 is supported on the upper curled edges of the Hakes 60. Both sides of plate 62 are ground and polished so that its upper surface 62a can also be used for Hattening purposes; thus a plurality of Hakes are also-disposed thereon. This can continue with additional plates, as Vshown at 64 and 66 as desired. A rod 68 of ceramic material is inserted through the passage formed by the holes 14a and 12a in metal covers'14 and 12 and lthe'hole-30b in top 30 of the refractory cubical for purposes to be hereinafter described.

When the plates and-,the Hakes have been stacked in the furnace as described above and the removable portion of the furnace'whichincludes the sides 32 and top 30 of the refractory cubical has been set in place, the temperature of the furnace is raised by energizing wire 42 to between 800 and 1050 C. The ceramic rod 68 is lowered -into the furnace, but before it touches the upper plate 66, it is allowed to reach the same temperature as the plates. When the rod reaches this temperature, the end is placed against the top plate 66, and a weight of approximately 1 lb. is hung on it and allowed to remain for between 3 and 5 minutes before removal. A higher pressure may be applied tothe rod when the weight is applied and removed, as by'the operator giving a Hrm push on the rod. Alternatively, a muchhigher pressure may be applied by a hydraulic press for a short period of time such as a few seconds. If this latter method of pressure application is used,it is unnecessary to apply a low pressure for a longer period such as 3-5 minutes. The Hakes become ductile at the temperatures mentioned and are plastically deformed by the pressure applied to them.; thus they assume the shape of the plates between which they are inserted. During heating and cooling of .the furnace'the pressure that can be applied to the plate-Hake stack is limited since ythe Hakes must expand and contract with respect to the plates. After the rod is removed, the furnace is shut off and the inner chamber is allowed to cool rapidly because of the lack of insulating material between .covers 12 and 14. Although the furnace may be allowed to cool to room temperature, it can be safely opened when the temperature has dropped to about 800 C. When the Hakes are removed from the furnace at the conclusion of this process, the great majority'will be Hat to within less than 10 microns, and few, if any, will be broken.

Although the inventionhas been described with respect to Hattening .curled Hakes resulting from the sintering process, if theplate upon which Ythe Hakes are placed is curved and the ,upper plate is shapedtointeriit therewith, the Hakes will acquire ithis curved shape. V-shaped Hakes can also'be made by having a plurality of Vs cut out of the upper surface ofthelower .plate and shaping an upper plate tomatch the'notches in the lower plate.

Thus I have described an'improved method for precisely shaping sintered'Hakes o-f thermally sensitive material to adapt them for ruseiin high speed thermistor bolometers. In addition my method can be used to shape thermistor Hakes for attachment lto curved surfaces `with cement layers of minimum uniform thickness. This has been accomplished by placing the curled Hakes between `ground and polished surfaces of high temperature glass, fused silica or sapphire, heatingthe plates-and the Hakes to a temperature above that at which the Hakes will soften and applying pressure as herein-described to shape the Hakes to the surfaces of the plates between which they are sandwiched.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and since certain changes may be made in carrying out the above method without departing from Ithe scope of the invention, it is intended that all matter contained in the above description shall be interpreted .aS illustrative and not in a limiting sense.

It is also to be understood that the claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. A method of changing the shape of sintered flakes of thermally sensitive resistance material which comprises placing said flakes on a glass plate having a shaped surface and a coefficient of expansion similar to that of said flakes, placing a second glass plate having a similar coefhcient of expansion above said first-mentioned glass plate by supporting it on said flakes, heating said rst and said second plates with said flakes between them to a temperature above the flake softening temperature, applying pressure to the plates when said flakes are in a softened condition whereby said flakes assume the shape of said plate surfaces, and cooling said flakes while they remain between said plates.

2. A method of flattening curled and sintered flakes of resistance material which comprises placing said flakes on a first glass plate having a flat surface and a coefficient of expansion similar to that of said flakes, supporting a second flat glass plate of corresponding thermal expansion characteristics on said flakes, heating said flakes to above the flake softening temperature, applying pressure to said plates while said flakes are above the flake softening temperature, whereby said flakes are flattened, and cooling said flakes to room temperature while remaining between said plates.

3. A method of flattening curled sintered flakes of thermally sensitive resistance material having a predetermined coeflicient of expansion which comprises placing said flakes on a first glass plate having a flat surface, said plate being made of glass having a high percentage of silica and a similar coeflicient of expansion to that of said flakes, supporting a second flat surface glass plate having a corresponding coefficient of expansion on said flakes, heating said flakes to above the flake softening temperature, applying pressure to said flakes while said flakes are above the flake softening temperature, whereby said flakes are flattened, and cooling said flakes to room temperature while remaining between said plates.

-4. The method defined in claim 3 in which said plates are made of synthetic sapphire having a coeilicient of expansion approximating said predetermined expansion coefcient of said sintered flakes.

5. A method of changing the shape of sintered flakes of resistance material having a predetermined coeicient of expansion which comprises placing said flakes on a plate of refractory material having a shaped surface and a similar coeficient of expansion, placing a second plate of refractory material having a corresponding coefficient of expansion above said first-mentioned plate and supporting it on said flakes, heating said first and said second plates with said flakes between them to a temperature of at least 800 C., holding said flakes and said plates at said temperature while applying pressure to said plates, whereby said flakes assume the shape of said plate surfaces and cooling said flakes while they remain between said plates.

6. A method of flattening curled sintered flakes of thermally sensitive resistance material of predetermined thermal expansion characteristics which comprises placing said flakes on a first plate of refractory material having a flat surface and similar thermal expansion characteristics, supporting a second plate of refractory material having a flat surface and corresponding expansion characteristics on said flakes, heating said flakes to above the flake softening temperature, applying a pressure of not more than 5 lbs. to said plate for a period of not more than 5 minutes while said flakes are above the flake softening temperature whereby said flakes are flattened, and cooling said flakes while they remain between said plates.

7.'A method of flattening curled sintered flakes of resistance material having a predetermined thermal coeflicient of expansion, which comprises placing said flakes on a first flat surface plate of glass containing at least 96 percent of silica and having a thermal coefficient of expansion similar to that of said sintered flakes, supporting a second plate of similar glass having a flat surface on top of said flakes, heating said flakes and said plates to a temperature in the range between 800 C. and 1050 C., applying a pressure of not more than 5 lbs. to the said second glass plate for a period of between 3 and 5 minutes while said flakes are at said temperature, whereby said flakes are flattened and allowing said akes to cool to room temperature while remaining between said plates.

8. A method of flattening curled sintered flakes of resistance material having a predetermined thermal coefficient of expansion, which comprises placing a plurality of said flakes on a glass plate having a flat surface and a similar coefllcient of expansion, supporting a second glass plate having a flat surface on said flakes, heating said flakes to above the flake softening temperature, causing a ceramic rod heated to the same temperature as said plates to engage the second plate, applying an initial pressure of not more than 5 pounds for not more than 1/2 minute to said rod, thereafter applying a pressure of not more than l pound for not more than 5 minutes to said rod, and then applying a final pressure of not more than 5 pounds for not more than 1/2 minute, whereby said flakes are flattened, removing said rod from engagement with said second plate, and cooling said plates and said flakes to room temperature w 'le said flakes remain between said plates.

References Cited in the file of this patent UNITED STATES PATENTS 958,813 Kelly May 24, 1910 1,398,926 Waite Nov. 26, 1921 1,497,799 Slingluf June 17, 1924 1,787,460 Wilcox Jan. 6, 1931 1,942,251 Mains Ian. 2, 1934 2,440,187 Silverberg Apr. 20, 1948 2,470,461 Black May 17, 1949 2,482,698 Tillyer Sept. 20, 1949 FOREIGN PATENTS 520,076 France June 20, 1921 606,649 Great Britain Aug. 18, 1948 

